(a) Industrial Field of the Invention
The present invention relates to a novel heat-resistant resin paste which may be suitably used as an overcoat material for screen process printing and to its use in production of integrated circuit devices.
(b) Description of the Related Art
Generally, resin solutions themselves hardly exhibit thixotropy. Thixotropy is defined as the phenomenon that apparent viscosity of an object decreases temporarily by deformation of the object while the temperature is held constant, and it is a flow property essential to pastes for screen process printing, the viscosities of which are required to temporarily decrease under the high shear rate during printing so as to permit the pastes to flow, but required to return to the original after the pastes are transferred to base materials so as to prevent the pastes from running or flowing. One method of endowing resin solutions with thixotropy is to turn the resin solutions into pastes by dispersing fine particles of resins as fillers in the resin solutions. There are known various such pastes.
Known resin solutions for the usages requiring not so high heat resistance are resin solutions of, for example, rosin-modified phenolic resins, rosin-modified maleic resins, melamine resins, and epoxy resins, and known resin solutions for the usages requiring high heat resistance include resin solutions of highly heat-resistant resins, for example, polyamic acid resins, which are precursors of polyimide resins, and certain polyimide resins, polyamide-imide resins, and polyamide resins which are soluble to solvents. As to the fine particles of resins to be dispersed in these resin solutions to form pastes, known ones for the usages requiring not so high heat resistance include, for example, fine particles of aliphatic polyamide resins, fine particles of melamine resins, fine particles of epoxy resins, and fine particles of phenolic resins, and known ones for the usages requiring high heat resistance include fine particles of highly heat-resistant resins, for example, fine particles of polyimide resins, fine particles of polyamide-imide resins, and fine particles of polyamide resins.
In production of semiconductive devices and wiring boards, insulator layers and surface protecting layers are generally formed by employing a screen process printing technique, and these insulator layers and surface protecting layers require high heat resistance, flexibility, moisture resistance, and corrosion resistance. For such usages, there have been developed screen process printing pastes prepared by dispersing fine particles of inorganic or organic matters as fillers in the above-described highly heat-resistant resin solutions. Inorganic fine particles however damage the flexibility which is innate in resins, because they themselves are hard and occupy large volume percentages in pastes because of their large gravities. Insufficient flexibility tends to cause cracks in films, and inorganic fine particles easily damage the surfaces of semiconductive devices, and therefore, lack reliability has been inevitable in insulator layers and protecting layers formed from pastes containing inorganic fine particles.
On the other hand, organic fine particles excelling in flexibility have been under investigation as the particles which may possibly solve the above-described problems. However, when dispersed as fillers in a film, they tend to leave vacant spaces between their surfaces and the resin which serves as an adhesive, thereby directly causing decreases in flexibility, moisture resistance, and corrosion resistance. Such a defect appears more severely in case of pastes prepared by using inorganic fine particles which have poor affinity with resins. Thus, the films formed from the conventional pastes, which have left the fine-particle fillers as they are in the films, have not been uniform and have had a tending to contain vacant spaces regardless of whether the fillers are inorganic fine particles or organic fine particles, and therefore, such films have not been necessarily satisfactory for usages requiring high flexibility, moisture resistance, and corrosion resistance.